1. Field of the Invention
The present invention relates to an apparatus and a method for drying an object to be treated, such as a semiconductor wafer (hereinafter referred to "wafer"), after the object is washed.
2. Description of the Related Art
In an apparatus for drying a washed object to be treated such as a wafer, a predetermined treatment liquid, e.g. isopropyl alcohol (IPA), having volatility is heated and evaporated in a bottom portion of a treatment bath. Using the vapor of IPA, water adhering to the surface of the object is removed. The water is received in a receiver situated in the treatment bath. The water is exhausted from the receiver to a drain tank outside the treatment bath via an exhaust pipe. A cooling spiral pipe is provided at an upper part of the treatment bath along a side wall of the treatment bath, in order to prevent the IPA vapor from escaping to the outside of the treatment bath. The IPA vapor is condensed (liquefied) by the cooling spiral pipe, and the IPA liquid drops are restored to a treatment liquid reservoir provided at a bottom portion of the treatment bath.
Treatment liquid drops produced by condensation of vapor of the treatment liquid are also put in the receiver. The temperature of the atmosphere within the treatment bath is kept at a substantially constant temperature, e.g. about 80.degree. C. However, the outside heat is transmitted to the receiver via the exhaust pipe, and the treatment liquid vapor near the receiver is cooled and condensed. The condensation of the treatment liquid near the receiver occurs not only during the drying of the object but also during the standby state of the drying process (water elimination). The conventional substrate drying apparatus of the type in which a liquid is exhausted from the receiver via the exhaust pipe. Thus, the treatment liquid in the receiver is exhausted equally during the drying of the object and the standby state.
However, the treatment liquid put in the receiver during the drying of the object is not mixed with water and not contaminated. If this treatment liquid is exhausted, it is wasted. In fact, this type of substrate drying apparatus is often operated 24 hours per day. Of the 24 hours, the actual operation time (in which the drying process is carried out) is about 8 hours, and the other hours are used in the standby state. Even if the amount of treatment liquid exhausted uselessly from the receiver during the standby time is small, a considerable amount of such treatment liquid, e.g. about 20.sub..h, is exhausted in a day.
In addition, in the conventional substrate drying apparatus, a gutter for receiving treatment liquid drops naturally falling from the cooling spiral pipe is provided just below the cooling spiral pipe along the side wall of the treatment bath. The treatment liquid received in the gutter is let to flow through a gap between the gutter and the inner wall of the treatment bath to the treatment liquid reservoir in the bottom portion of the treatment bath. However, in this kind of treatment liquid recovery method, the treat liquid to be recovered, along with contamination, dust, etc. adhering to the gutter and the inner wall of the treatment bath, flows to the treatment liquid reservoir. Thus, the purity of the treatment liquid tends to deteriorate. Accordingly, the treatment liquid must be changed frequently. Furthermore, there is a concern that the treatment liquid falling through the gap between the gutter and the inner wall of the treatment bath is dispersed and adhered to the object, receiver, etc.